1. Field of the Invention
This invention relates to an electronic circuit and more particularly to a comparator circuit for comparing two input signals and producing a resulting output signal.
2. Background of the Relevant Art
A comparator circuit used for comparing two input signals is well known. When operating in its nonlinear range, a comparator provides digital output voltage information corresponding to the difference between the input voltages. For example, if the input voltage placed on the non-inverting input is greater than the voltage placed on the inverting input, then a relatively large output signal may be produced. Conversely, if the input voltage on the non-inverting input is lower than the input voltage on the inverting input, then a relatively low output voltage may be produced. Accordingly, the comparator functions to compare two input signals and produce a digital output based upon that comparison.
Most comparator circuits include at least two stages of operation. For example, typical comparators utilize a differential amplifier or differential transistor pair at the input stage of operation and a plurality of loads/buffers at the output stage. The amplifier and load circuits receive biasing from constant current sources and voltage references generally produced on the same monolithic circuit as that of the comparator. By integrating the reference voltage devices and constant current sources on the same silicon chip as the comparator, fewer package leads are necessary--i.e., power need not be imported off-the-chip. Furthermore, integration ensures matched operation of each component due to fairly uniform process parameters occurring across the substrate surface.
A problem often encountered with many conventional comparators is the detrimental effects of noise upon the input signals feeding the comparator. For example, if one input slightly exceeds the other due to noise, then the output may inadvertently trigger to an opposing digital value. Inadvertent triggering of the output may occur in rapid succession due to rapid and slight fluctuations of the input signal noise. The existence of such fluctuations in the comparator's output caused by input noise is sometimes referred to as "chatter."
In an effort to overcome chatter problems, many comparators utilize a hysteresis feedback loop from the comparator's output to the input amplifier stage. A comparator circuit which uses a feedback hysteresis loop is described in U.S. Pat. No. 4,670,671. Patent '671 describes a comparator circuit having two separate differential amplifiers, wherein one amplifier receives feedback information from the output of the comparator and the other amplifier receives the input signals. The device of Patent '671 ensures that the output will not change unless one input signal exceeds the other input signal by a hysteresis value. Unless the noise peaks on one input exceed the other input by the hysteresis value, the comparator's output will not inadvertently toggle or chatter.
The comparator of Patent '671 is primarily designed to provide hysteresis feedback during high speed operation while using relatively little power. As such, each differential amplifier (feedback and input differential amplifiers) of Patent '671 requires a pair of feedback current mirrors or loads. While these loads provide high speed operation with low power consumption, each load must utilize three transistors interconnected with positive and negative driver circuits. The inclusion of separate loads and drivers adds to the overall complexity of the device by requiring numerous transistors and associated interconnect routing. The additional transistors and routing merely adds to the cost in manufacturing a monolithic device as well as reduces reliability of the device.